This invention relates generally to storage tanks and, more particularly, to a frangible roof joint for such tanks, especially smaller tanks (e.g., those having a diameter of less than 50 feet (15.2m)).
In the event of a rapid pressure build-up in a storage tank, as during a fire, for example, it is important that there be emergency venting provisions to avoid rupture of the tank and spillage of its contents especially where the material being stored is flammable or combustible. One way of providing emergency venting is a frangible roof joint, which is a joint between the roof and the shell of a storage tank designed to fail at relatively low pressures. Failure of the joint provides an opening sufficiently large to vent the tank, thereby avoiding rupture of the shell itself or the floor-to-shell joint of the tank which would result in spillage.
The American Petroleum Institute (API) has developed a standard for the design and construction of welded steel storage tanks. This standard provides for the design of a frangible roof joint, but in 1978 the standard was amended to limit applicability of the frangible roof design to tanks having a diameter of fifty feet (15.2m) or more. This was done to alert the industry that frangible roof designs then available were generally not considered effective in small-diameter tanks. Moreover, the API frangible roof design standard has been further limited to tanks having a conical roof where the ratio of rise to run of the roof does not exceed two inches (51mm) in twelve (305mm). The standard is not applicable to domed roofs.
Thus there is a present need in the industry for a frangible roof design for relatively small-diameter tanks (e.g., tanks less than fifty feet (15.2m) in diameter) and for tanks having domed roofs or conical roofs where the rise of the roof exceeds the run of the roof by more than two inches (51mm) in twelve (305mm).